Merge tag 'dma-mapping-6.7-2023-10-30' of git://git.infradead.org/users/hch/dma-mapping

Pull dma-mapping updates from Christoph Hellwig:

 - get rid of the fake support for coherent DMA allocation on coldfire
   with caches (Christoph Hellwig)

 - add a few Kconfig dependencies so that Kconfig catches the use of
   invalid configurations (Christoph Hellwig)

 - fix a type in dma-debug output (Chuck Lever)

 - rewrite a comment in swiotlb (Sean Christopherson)

* tag 'dma-mapping-6.7-2023-10-30' of git://git.infradead.org/users/hch/dma-mapping:
  dma-debug: Fix a typo in a debugging eye-catcher
  swiotlb: rewrite comment explaining why the source is preserved on DMA_FROM_DEVICE
  m68k: remove unused includes from dma.c
  m68k: don't provide arch_dma_alloc for nommu/coldfire
  net: fec: use dma_alloc_noncoherent for data cache enabled coldfire
  m68k: use the coherent DMA code for coldfire without data cache
  dma-direct: warn when coherent allocations aren't supported
  dma-direct: simplify the use atomic pool logic in dma_direct_alloc
  dma-direct: add a CONFIG_ARCH_HAS_DMA_ALLOC symbol
  dma-direct: add dependencies to CONFIG_DMA_GLOBAL_POOL

1 files changed, 7 insertions, 5 deletions

diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index dff067bd56b1..26202274784f 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -1301,11 +1301,13 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
 		pool->slots[index + i].orig_addr = slot_addr(orig_addr, i);
 	tlb_addr = slot_addr(pool->start, index) + offset;
 	/*
-	 * When dir == DMA_FROM_DEVICE we could omit the copy from the orig
-	 * to the tlb buffer, if we knew for sure the device will
-	 * overwrite the entire current content. But we don't. Thus
-	 * unconditional bounce may prevent leaking swiotlb content (i.e.
-	 * kernel memory) to user-space.
+	 * When the device is writing memory, i.e. dir == DMA_FROM_DEVICE, copy
+	 * the original buffer to the TLB buffer before initiating DMA in order
+	 * to preserve the original's data if the device does a partial write,
+	 * i.e. if the device doesn't overwrite the entire buffer.  Preserving
+	 * the original data, even if it's garbage, is necessary to match
+	 * hardware behavior.  Use of swiotlb is supposed to be transparent,
+	 * i.e. swiotlb must not corrupt memory by clobbering unwritten bytes.
 	 */
 	swiotlb_bounce(dev, tlb_addr, mapping_size, DMA_TO_DEVICE);
 	return tlb_addr;